The present invention pertains to vertical cavity surface emitting lasers (VCSEL""s) and particularly to VCSEL""s made by a metal-organic chemical vapor deposition (MOCVD) process.
The perspective view shown in FIG. 1 illustrates a typical structure for a vertical cavity surface emitting laser 10. A gallium arsenide substrate 12 is disposed on an n type electrical contact 14. A first mirror stack 16 and a bottom graded index region or lower spacer 18 are progressively disposed, in layers, on the substrate 12.
Region 20 may have one or many quantum wells or may be a bulk active gain region. An active region 20, having one or more quantum wells, is formed and a top graded index region or upper spacer 22 is disposed over active region 20. The spacers are to provide the appropriate critical distance between the mirrors to provide the proper-sized resonant cavity for a given wavelength and the distance is related to that wavelength or a multiple thereof. Active region 20 has a gain that compensates for the leaking out of photons. Photons bounce back and forth and, due to ),imperfect mirrors 16 and 24, eventually leak out of the device. Greater photon loss means more gain is needed.
A p type top mirror stack 24 is formed over active region 20 and a metal layer 26 forms an electrical contact. Current 21 can be caused to flow from the upper contact 26 to the lower contact 14. This current 21 passes through the active region 20. Upward arrows in FIG. 1 illustrate the passage of light 23 through an aperture or hole 30 in the upper metal contact 26. Downward arrows illustrate the passage of current 21 downward from the upper contact 26 through p type GaAs cap layer 8, p type conduction layer 9, p type upper mirror stack 24 and active region 20. A hydrogen ion bombardment or implantation 40 forms an annular region of electrically resistant material. In order to confine the current flow 21 through active region 20, device 10 uses a hydrogen ion implant technique to create electrically insulative regions around an electrically conductive opening extending therethrough. A central opening 42 of electrically conductive material remains undamaged during the ion implantation process. As a result, current 21 passing from upper contact 26 to lower contact 14 is caused or forced to flow through electrically conductive opening 42 and is thereby selectively directed or confined to pass through a preselected portion of active region 20.
The present problem concerns active region 20 of the device. The issue relates to the reliability implications that result from the interaction between carbon and hydrogen in the VCSEL structure. There have been vertical cavity surface emitting lasers that have had short term degradation caused by hydrogen passivation or compensation-of carbon. Hydrogen compensates carbon acceptors in AlGaAs. This phenomenon is a byproduct of the MOCVD growth process and also results from proton implantation. Carbon ions are used in doping. Carbon doping brings in a significant amount of hydrogen. The results of hydrogen passivation are rapid degradation of the devices sometimes followed by rapid improvement, which is the result of the hydrogen moving through the structure under bias. Longer baking during the fabrication process drives out more hydrogen.
There are several kinds of doped structures. If low doping xe2x89xa65xc3x971017/cm3 (5el7) (curve 13 in FIG. 2a) is used in p-spacer 22 of FIGS. 1 and 3, then the structure is sensitive to mobile hydrogen. The sensitivity to mobile hydrogen occurs because hydrogen acts as a donor and compensates the carbon. However, the hydrogen is very mobile and under field-aided diffusion, hydrogen H drifts towards active region 20 and compensates carbon C acceptors on the edge of active region 20 in FIG. 2a. One way to overcome this compensation is to use higher p doping near active region 20 (see curve 15 in FIG. 2a). The problem that arises with such doping is that there remains a large slope 19 in active region 20 even at lasing voltages in FIG. 2c. The separation of the carriers resulting from energy band slope versus position 19 makes the recombination inefficient. To overcome this problem, a thick (15 nanometers) effectively undoped region is placed on lower side 18 of active region 20. The voltage is then allowed to drop across the undoped region.
The post growth anneal and the use of low arsine over pressures during growth have been other solutions attempted to prevent hydrogen passivation or compensation of carbon that causes at least short term degradation of VCSEL""s. The present invention is a structural solution to the problem.
Schneider et al, U.S. Pat. No. 5,557,627, discloses a visible VCSEL that allows the use of Carbon as a p-type dopant, which improves the dopant precision and stability against thermal diff-usion during epitaxial growth and subsequent device processing. To allow the use of Carbon, a barrier or spacer layer is fabricated on the p-side of the active region, having little or no indium.
In Kobayashi et al, U.S. Pat. No. 5,513,202, a VCSEL having a substrate, a p-type mirror, a p-type spacer layer, an active region, an n-type spacer layer, and an n-type mirror is disclosed. The problem addressed in this patent concerns the high vertical resistance in a non-graded p-type mirror. One embodiment disclosed involves a p-type spacer layer with an impurity concentration higher than that of the p-type bottom mirror. More specifically, Kobayashi discloses that the p-type spacer layer has an impurity concentration of 3xc3x971018 cmxe2x88x923. Additionally, the n-type spacer layer has a concentration of 3xc3x971018 cmxe2x88x923, but may also be an undoped layer.
A structural way to make the VCSEL structure 10 less sensitive to the hydrogen passivation problem is to use heavily doped layers near active region 20 (of FIGS. 1 and 3). These layers would be too heavily doped for the hydrogen to completely compensate. If this doping is not carefully performed, device 10 will not work because energy band structure 17 of active region 20 will have a residual tilt 19 even at lasing voltages (as shown in FIG. 2c). This represents an electric field across active region 20. The electric field causes the carriers of the opposite charge to preferentially seek one side or the other of active region 20 and radiative recombination becomes inefficient. Since radiative recombination is inefficient, parasitic recombination mechanisms dominate.
To eliminate this residual tilt of bands 17, an effectively undoped section 18 in the n graded region (or close to active region 20 on the n-side which may include an n spacer) must be present and this undoped region 18 must be of sufficient extent. The term xe2x80x9ceffectively undopedxe2x80x9d means that there are residual impurities in any material and that there is in reality no such thing as strictly undoped material. For purposes in this description and the claims, xe2x80x9cunintentionally doped,xe2x80x9d xe2x80x9ceffectively undopedxe2x80x9d and xe2x80x9cundopedxe2x80x9d mean the same thing and may be used here interchangeably. Additionally, p doping can be added to active region 20. Two dissimilar materials (i.e., n and p doped), when placed together, have different work functions and charges that flow from one to another. So before a bias voltage for lasing is applied, there is a built-in voltage between the p and n regions. This voltage causes an electric field/energy band slope which is reduced by reducing the charge at the junction. This is accomplished by introducing an undoped (uncharged) region. As voltage is applied the band flattens further and the electric field is reduced.
In summary, the invention has two features. It provides for relatively high doping in the p regions 22 down to and optionally through active region 20, and it has a thick undoped region in the lower graded region 18. The high p-doping in spacer 22 makes the structure insensitive to hydrogen, and the thick lower undoped region 18 allows the electric field to drop across this region making the active region energy wells relatively flat at lasing voltages (in FIG. 4c). An alternate structure may be that in the above, the p doping and regions be interchanged with the n doping and regions.
In essence, to eliminate sensitivity to mobile hydrogen, a heavily doped region (i.e., in the upper spacer) needs to be placed adjacent to the active region. To reduce the tilt of the energy bands in the active region (under active lasing bias voltage across the VCSEL) resulting from the heavily doped region adjacent to the active region, an undoped or unintentionally doped region (i.e., in the lower spacer) of sufficient extent is placed on the other end of the active region.